Short optical pulses are useful for numerous scientific and engineering applications: a partial list includes optical communication, time-resolved spectroscopy, optical radar, metrology, sampling and clock signal generation. There is special interest in laser-generated pulses because of their potentially high intensities, short durations and coherence properties.
Historically, means of generating short optical pulses have consisted of (a) amplitude (gain or loss) modulation or switching of a steady or quasi-steady source such as a lamp or a laser (continuous-wave (CW) or pulsed), or (b) mode-locking a laser (CW or pulsed). The latter technique, mode-locking, refers to a variety of regenerative schemes whereby one or more optical pulses is induced to circulate in a laser resonator in which it is narrowed, conditioned and stabilized over many complete round trips.
Gain/loss modulation schemes are relatively straightforward but require bulky and expensive external signal generators. These electronic signal generators are employed to produce sinusoidal waves or pulse trains which are then amplified and used to modulate the light source. The pulses produced using these techniques are relatively long in duration even when lasers are employed as sources. The mode-locking schemes do produce short pulses (the shortest pulses, .about.10 fs, have been generated by mode-locking tunable lasers such as dye and titanium-sapphire devices) and can in some instances be made compact and inexpensive, but the mode-locking processes are very subtle and occur only h restricted parameter ranges, so that the pulse amplitudes are not easily variable. Also, since the pulse repetition rate equals the inverse of the optical round trip delay time in the laser resonator, tuning this repetition rate continuously is extremely difficult because it requires adjusting the optical length of the laser resonator while preserving perfect alignment. Moreover, because of the complicated dynamical nature of mode-locking involving an interplay between several laser and external parameters, mode-locked lasers are difficult to stabilize over extended periods (hours).
A hybrid technique, known as regenerative pulse generation, has been implemented using semiconductor diode lasers. As explained in "Frequency Stabilization and Narrowing of Optical Pulses from CW GaAs Injection Lasers", by T. L. Paoli and J. E. Ripper (IEEE Journal of Quantum Electronics, Vol. QE-6, p. 335, June 1970) and U.S. Pat. No. 3,641,459, a portion of the output light is detected to produce an electrical signal which is then amplified and used to stabilize the self-sustained pulsations emanating from the laser. In this technique the repetition rate of the pulsations is equal to the relaxation oscillation frequency of the laser diode, hence the pulse repetition rate has only limited tunability and limited choices of the center frequency of the tuning range, and changing the pulse repetition rate almost always results in changing the pulse amplitude considerably. Moreover, the existence of sustained self-pulsations is detrimental to the long-term reliability of the laser diode and these pulsations are considered undesirable.
In U.S. Pat. No. 3,617,932 entitled "Method for Pulse-Width-Modulating Semiconductor Lasers", by T. L. Paoli and J. E. Ripper, there is described a scheme in which laser diodes exhibiting sustained self-pulsations at constant pulse repetition rates are modulated by converting a portion of the output light into an electrical signal using a photodiode, then amplifying this signal and using it to modulate the existing pulsations. Again this technique uses lasers exhibiting self-sustained pulsations which have the same undesirable limitations as those discussed in the previous paragraph.
In "Optoelectronic Regenerative Pulser", by T. C. Damen and M. A. Duguay (Electronics Letters, Vol. 16, p. 166, Feb. 1980), them is described a regenerative pulse generation scheme in which a portion of the light output is convened into an electrical signal by a photodetector, this electrical signal then being amplified and applied directly without deliberate filtering to modulate the drive current to the diode laser. In this scheme the repetition rate is governed by the total feedback loop delay (i.e. the sum of the signal propagation delays in the optical and electrical portions of the loop) and the resulting optical pulses are unstable and noisy. While the pulse amplitude could be adjusted within broad limits, tuning of the pulse repetition rate is difficult, involving variation of the optical and/or electrical delay time. Moreover, initiation of the pulsations is achievable only by a relatively cumbersome process of first increasing the laser bias current to a "turn-on" value and then reducing it to the steady-state operating value.
In "Self-Sustained Picosecond Pulse Generation in a GaAlAs Laser at an Electrically Tunable Repetition Rate by Optoelectronic Feedback", by K. Y. Lau and A. Yariv (Applied Physics Letters, Vol. 45, p. 124, July 1984), them is discussed a technique involving application of similar optoelectronic feedback to a diode laser exhibiting sustained self-pulsations. In this arrangement the pulse repetition rate is tuned by varying the self-pulsing frequency, which is accomplished by adjusting the drive current to the laser diode. This scheme involves undesirable self-pulsations as discussed in the previous paragraphs, and varying the pulse repetition rate is difficult and usually necessitates changing the pulse amplitude and pulse width.